Polyimide films are well known in the art. Chemically, such films are composed of a polymer having the recurring unit (1) ##STR1## wherein: R is an organic tetravalent radical containing at least two carbon atoms, no more than two carbonyl groups being attached to any one carbon atom of R; and
R' is a divalent radical containing at least two carbon atoms, each one of the two valencies of R' being on a separate carbon atom.
Various polyimides and their methods of manufacture are described, among others, in the following U.S. Pat. Nos.: 3,179,614 (Edwards); 3,179,634 (Edwards); 3,179,633 (Endrey); and 3,316,212 (Angelo et al.).
The polyimide normally is made by dehydrating and cyclizing a precursor polyamic acid, for example, as shown below: ##STR2## where n is a whole number.
The precursor polyamic acid is normally made from a dianhydride of a tetracarboxylic acid, e.g., pyromellitic acid, and a diamine, e.g., 4,4'-diaminodiphenyl ether, by simple condensation at room or lower temperature in a solvent in which both the starting materials and the resulting polyamic acid are soluble, for example, N,N-dimethylacetamide (sometimes hereinafter abbreviated to DMAC). In order to obtain a polyimide film, the precursor solution according to one technique is cast on a substrate, such as a metal or glass plate, and heated to a sufficiently high temperature to remove substantially all of the solvent, and to cause water elimination and cyclization, as shown in the above equation, e.g., 300.degree.-360.degree. C. See, for example, the above cited U.S. patents. This is known as thermal conversion of polyamic acid to polyimide. According to another technique, known as chemical conversion, dehydrating chemicals such as a combination of acetic anhydride and pyridine are first added to the polyamic solution before casting. This permits conversion of polyamic acid to polyimide at a lower temperature or, alternatively, in a shorter time. Depending on the particular polyimide, the solvent, the film thickness, and the temperature, some solvent may be retained in the polyimide film whichever technique is used. In the case of certain commercial polyimide films which are used in such applications as circuit boards for computers and in similarly sensitive electronic equipment whose performance could be affected by anything more than trace amounts of the solvent, it is usually necessary to dry the film at a rather high temperature (about 360.degree. C. or more) for a rather long time (often several minutes) in order to obtain a product of sufficiently high quality.
It would be desirable to be able to effect good solvent removal either at a lower temperature or within a shorter time or, even more so, to a higher degree of dryness.